Synthesis and Biological Properties of Alanine-Grafted Hydroxyapatite Nanoparticles

Hydroxyapatite attracts great attention as hard tissues implant material for bones and teeth. Its application in reconstructive medicine depends on its biocompatibility, which is in a function of composition and surface properties. The insertion of a protein element in the composition of implants can improve the cell adhesion and the osseointegration. Having this in mind, the proposal of this work was to develop L-alanine-grafted hydroxyapatite nanoparticles and to study their biocompatibility. Two L-alanine sources and three grafting methods were used for hydroxyapatite surface functionalization. The efficiency of grafting was determined based on X-ray powder diffraction, Fourier-transform infrared spectroscopy, thermal analyses, and field-emission scanning electron microscopy. The results indicated the formation of hydroxyapatite with 8-25 wt% of organic content, depending on the grafting method. Protein adsorption, cell adhesion, and viability studies were carried out to evaluate biological properties of grafted materials. The viability of MG-63 human osteoblastic cells following 24 h incubation with the alanine-grafted hydroxyapatite samples is well preserved, being in all cases above the viability of cells incubated with hydroxyapatite. The alanine-grafted hydroxyapatite prepared in situ and by simple mixture showed higher protein adsorption and cell adhesion, respectively, indicating their potential toward use in regenerative medicine.

Bio-Inspired Proanthocyanidins from Blueberries’ Surface Coating Prevents Red Blood Cell Agglutination on Urinary Silicon-Based Catheters

Thrombosis can cause the occlusion of implantable medical devices, leading to the rejection of the device and subsequent mortality. Thrombosis is primarily induced by red blood aggregation and coagulation. The administration of anticoagulant drugs is generally used as a treatment to avoid these processes. Adverse effects such as bleeding in the event of an anticoagulant overdose, osteoporosis associated with prolonged use, hypersensitivity, and hives have been reported. New strategies such as biomolecule surface functionalization have recently been studied to overcome these problems. In this study, we report a novel coating composed of polydopamine (PDA) and proanthocyanidins (PACs) from blueberry extract to avoid red blood aggregation in short-term use medical devices such as silicone catheters. We showed that PDA formed stable films on silicone surfaces and PACs could be immobilized on PDA layers using laccase as a catalyst. The PDA–PACs films decreased surface hydrophilicity, increased surface roughness, and decreased plasma protein adsorption. The films were stable in phosphate buffer saline (PBS) and cell culture media. Furthermore, red blood cell adsorption and aggregation decreased. These effects are attributed to changes in the membrane fluidity that influences adhesion, the steric hindrance of the layers, and the low adsorption of plasma proteins on the PAC layer.

The influence of different rewetting procedures on the thrombogenicity of nanoporous poly(ether imide) microparticles

Nanoporous microparticles prepared from poly(ether imide) (PEI) are discussed as candidate adsorber materials for the removal of uremic toxins during apheresis. Polymers exhibiting such porosity can induce the formation of micro-gas/air pockets when exposed to fluids. Such air presenting material surfaces are reported to induce platelet activation and thrombus formation. Physical or chemical treatments prior to implantation are discussed to reduce the formation of such gas nuclei. Here, we report about the influence of different rewetting procedures - as chemical treatments with solvents - on the thrombogenicity of hydrophobic PEI microparticles and PEI microparticles hydrophilized by covalent attachment of poly(vinyl pyrrolidone) (PVP) of two different chain lengths.Autoclaved dry PEI particles of all types with a diameter range of 200 - 250 μm and a porosity of about 84% ±2% were either rewetted directly with phosphate buffered saline (24 h) or after immersion in an ethanol-series. Thrombogenicity of the particles was studied in vitro upon contact with human sodium citrated whole blood for 60 min at 5 rpm vertical rotation. Numbers of non-adherent platelets were quantified, and adhesion of blood cells was qualitatively analyzed by bright field microscopy. Platelet activation (percentage of CD62P positive platelets and amounts of soluble P-Selectin) and platelet function (PFA100 closure times) were analysed.Retention of blood platelets on the particles was similar for all particle types and both rewetting procedures. Non-adherent platelets were less activated after contact with ethanol-treated particles of all types compared to those rewetted with phosphate buffered saline as assessed by a reduced number of CD62P-positive platelets and reduced amounts of secreted P-Selectin (P < 0.05 each). Interestingly, the hydrophilic surfaces significantly increased the number of activated platelets compared to hydrophobic PEI regardless of the rewetting agent. This suggests that, apart from wettability, other material properties might be more important to regulate platelet activation. PFA100 closure times were reduced and within the reference ranges in the ethanol group, however, significantly increased in the saline group. No substantial difference was detected between the tested surface modifications. In summary, rewetting with ethanol resulted in a reduced thrombogenicity of all studied microparticles regardless of their wettability, most likely resulting from the evacuation of air from the nanoporous particles.

Response of Endothelial Cells to Gelatin-Based Hydrogels

The establishment of confluent endothelial cell (EC) monolayers on implanted materials has been identified as a concept to avoid thrombus formation but is a continuous challenge in cardiovascular device engineering. Here, material properties of gelatin-based hydrogels obtained by reacting gelatin with varying amounts of lysine diisocyanate ethyl ester were correlated with the functional state of hydrogel contacting venous EC (HUVEC) and HUVEC's ability to form a monolayer on these hydrogels. The density of adherent HUVEC on the softest hydrogel at 37 °C (G' = 1.02 kPa, E = 1.1 ± 0.3 kPa) was significantly lower (125 mm^-1) than on the stiffer hydrogels (920 mm^-1; G' = 2.515 and 5.02 kPa, E = 4.8 ± 0.8 and 10.3 ± 1.2 kPa). This was accompanied by increased matrix metalloprotease activity (9 pmol·min^-2 compared to 0.6 pmol·min^-2) and stress fiber formation, while cell-to-cell contacts were comparable. Likewise, release of eicosanoids (e.g., prostacyclin release of 1.7 vs 0.2 pg·mL^-1·cell^-1) and the pro-inflammatory cytokine MCP-1 (8 vs <1.5 pg·mL^-1·cell^-1) was higher on the softer than on the stiffer hydrogels. The expressions of pro-inflammatory markers COX-2, COX-1, and RAGE were slightly increased on all hydrogels on day 2 (up to 200% of the control), indicating a weak inflammation; however, the levels dropped to below the control from day 6. The study revealed that hydrogels with higher moduli approached the status of a functionally confluent HUVEC monolayer. The results indicate the promising potential especially of the discussed gelatin-based hydrogels with higher G' as biomaterials for implants foreseen for the venous system.

Smart Molecular Nanosheets for Advanced Preparation of Biological Samples in Electron Cryo-Microscopy

Transmission electron cryo-microscopy (cryoEM) of vitrified biological specimens is a powerful tool for structural biology. Current preparation of vitrified biological samples starts off with sample isolation and purification, followed by the fixation in a freestanding layer of amorphous ice. Here, we demonstrate that ultrathin (∼10 nm) smart molecular nanosheets having specific biorecognition sites embedded in a biorepulsive layer covalently bound to a mechanically stable carbon nanomembrane allow for a much simpler isolation and structural analysis. We characterize in detail the engineering of these nanosheets and their biorecognition properties employing complementary methods such as X-ray photoelectron and infrared spectroscopy, atomic force microscopy as well as surface plasmon resonance measurements. The desired functionality of the developed nanosheets is demonstrated by in situ selection of a His-tagged protein from a mixture and its subsequent structural analysis by cryoEM.

Low Fouling, Peptoid-Coated Polysulfone Hollow Fiber Membranes-the Effect of Grafting Density and Number of Side Chains

The development of low fouling membranes to minimize protein adsorption has relevance in various biomedical applications. Here, electrically neutral peptoids containing 2-methoxyethyl glycine (NMEG) side chains were attached to polysulfone hollow fiber membranes via polydopamine. The number of side chains and grafting density were varied to determine the effect on coating properties and the ability to prevent fouling. NMEG peptoid coatings have high hydrophilicity compared to unmodified polysulfone membranes. The extent of biofouling was evaluated using bovine serum albumin, as well as platelet adhesion. The results suggest that both the number of side chains and grafting density play a role in the surface properties that drive biofouling. Protein adsorption decreased with increasing peptoid grafting density and is lowest above a critical grafting density specific to peptoid chain length. Our findings show that the optimization of grafting density and hydration of the surface are important factors for achieving the desired antifouling performance.

Dynamic Protein Adsorption onto Dendritic Polyglycerol Sulfate Self-Assembled Monolayers

Biomaterial surfaces that are in contact with blood are often prone to unspecific protein adsorption and the activation of the blood clotting cascade. Hence, such materials usually must be functionalized with low-fouling or anticoagulant polymer coatings to increase their performance and durability with respect to various applications, for example as implants or in biomedical devices. Many coatings are based on anionic polymers, such as heparin, and are known to have pronounced anticoagulant effects. To assess the ability of a surface to prevent biofouling and to get further insight into its underlying mechanism, studies of the protein adsorption on self-assembled monolayers (SAMs) are often used as a predictive tool. In this article, we synthesized thioctic acid-functionalized dendritic polyglycerol sulfate (dPGS), which is a well-known synthetic heparin mimetic, and immobilized it onto gold model surfaces. The anionic dPGS SAMs were characterized via contact angle measurements and ellipsometry and compared to their neutral dendritic polyglycerol (dPG) counterparts with respect to their single protein adsorption of the two most abundant blood proteins albumin (Alb) and fibrinogen (Fib). In addition, we used QCM-D and ToF-SIMS as complementary techniques to investigate the dynamic, mixed, and sequential adsorption of Alb and Fib. Our results clearly demonstrate an incomplete Vroman effect and indicate the rearrangement of the adsorbed protein layers, which is presumably drive by ionic interactions between the two proteins and the anionic dPGS surface.

Tunable Elastomers with an Antithrombotic Component for Cardiovascular Applications

This study reports the development of a novel family of biodegradable polyurethanes for use as tissue engineered cardiovascular scaffolds or blood-contacting medical devices. Covalent incorporation of the antiplatelet agent dipyridamole into biodegradable polycaprolactone-based polyurethanes yields biocompatible materials with improved thromboresistance and tunable mechanical strength and elasticity. Altering the ratio of the dipyridamole to the diisocyanate linking unit and the polycaprolactone macromer enables control over both the drug content and the polymer cross-link density. Covalent cross-linking in the materials achieves significant elasticity and a tunable range of elastic moduli similar to that of native cardiovascular tissues. Interestingly, the cross-link density of the polyurethanes is inversely related to the elastic modulus, an effect attributed to decreasing crystallinity in the more cross-linked polymers. In vitro characterization shows that the antiplatelet agent is homogeneously distributed in the materials and is released slowly throughout the polymer degradation process. The drug-containing polyurethanes support endothelial cell and vascular smooth muscle cell proliferation, while demonstrating reduced levels of platelet adhesion and activation, supporting their candidacy as promising substrates for cardiovascular tissue engineering.

Albumin solder covalently bound to a polymer membrane: New approach to improve binding strength in laser tissue soldering in-vitro

Laser tissue soldering (LTS) based on indocyanine green (ICG)-mediated heat-denaturation of proteins might be a promising alternative technique for micro-suturing, but up to now the problem of too weak shear strength of the solder welds in comparison to sutures is not solved. Earlier reports gave promising results showing that solder supported by carrier materials can enhance the cohesive strength of the liquid solder. In these studies, the solder was applied to the carriers by dip coating. Higher reliability of the connection between the solder and the carrier material is expected when the solder is bound covalently to the carrier material. In the present study a poly(ether imide) (PEI) membrane served as carrier material and ICG-supplemented albumin as solder substrate. The latter was covalently coupled to the carrier membrane under physiological conditions to prevent structural protein changes. As laser source a diode continuous-wave laser emitting at 808 nm with intensities between 250 mW and 1500 mW was utilized. The albumin functionalized carrier membrane was placed onto the tunica media of explanted pig thoracic aortae forming an overlapping area of approximately 0.5×0.5 cm2. All tests were performed in a dry state to prevent laser light absorption by water. Infrared spectroscopy, spectro-photometrical determination of the secondary and primary amine groups after acid orange II staining, contact angle measurements, and atomic force microscopy proved the successful functionalization of the PEI membrane with albumin. A laser power of 450 mW LTS could generate a membrane-blood vessel connection which was characterized by a shear strength of 0.08±0.002 MPa, corresponding to 15% of the tensile strength of the native blood vessel. Theoretically, an overlapping zone of 4.1 mm around the entire circumference of the blood vessel could have provided shear strength of the PEI membrane-blood vessel compound identical to the tensile strength of the native blood vessel. These in-vitro results confirmed the beneficial effects of solder reinforcement by carrier membranes, and suggest LTS with covalently bound solders on PEI substrates for further studies in animal models.

Evaluation of platelet adhesion and activation on polymers: Round-robin study to assess inter-center variability

The regulatory agencies provide recommendations rather than protocols or standard operation procedures for the hemocompatibility evaluation of novel materials e.g. for cardiovascular applications. Thus, there is a lack of specifications with regard to test setups and procedures. As a consequence, laboratories worldwide perform in vitro assays under substantially different test conditions, so that inter-laboratory and inter-study comparisons are impossible. Here, we report about a prospective, randomized and double-blind multicenter trial which demonstrates that standardization of in vitro test protocols allows a reproducible assessment of platelet adhesion and activation from fresh human platelet rich plasma as possible indicators of the thrombogenicity of cardiovascular implants. Standardization of the reported static in vitro setup resulted in a laboratory independent scoring of the following materials: poly(dimethyl siloxane) (PDMS), poly(ethylene terephthalate) (PET) and poly(tetrafluoro ethylene) (PTFE). The results of this in vitro study provide evidence that inter-laboratory and inter-study comparisons can be achieved for the evaluation of the adhesion and activation of platelets on blood-contacting biomaterials by stringent standardization of test protocols.

Effect of temperature on platelet adherence

BACKGROUND

Thrombogenicity is one of the main parameters tested in vitro to evaluate the hemocompatibility of artificial surfaces. While the influence of the temperature on platelet aggregation has been addressed by several studies, the temperature influence on the adherence of platelets to body foreign surfaces as an important aspect of biomedical device handling has not yet been explored. Therefore, we analyzed the influence of two typically applied incubation-temperatures (22°C and 37°C) on the adhesion of platelets to biomaterials.

MATERIAL AND METHODS

Thrombogenicity of three different polymers - medical grade poly(dimethyl siloxane) (PDMS), polytetrafluoroethylene (PTFE) and polyethylene terephthalate (PET) - were studied in an in vitro static test. Platelet adhesion was studied with stringently characterized blood from apparently healthy subjects. Collection of whole blood and preparation of platelet rich plasma (PRP) was carried out at room temperature (22°C). PRP was incubated with the polymers either at 22°C or 37°C. Surface adherent platelets were fixed, fluorescently labelled and assessed by an image-based approach.

RESULTS AND DISCUSSION

Differences in the density of adherent platelets after incubation at 22°C and 37°C occurred on PDMS and PET. Similar levels of adherent platelets were observed on the very thrombogenic PTFE. The covered surface areas per single platelet were analyzed to measure the state of platelet activation and revealed no differences between the two incubation temperatures for any of the analyzed polymers. Irrespective of the observed differences between the low and medium thrombogenic PDMS and PET and the higher variability at 22°C, the thrombogenicity of the three investigated polymers was evaluated being comparable at both incubation temperatures.

Adhesion and activation of platelets from subjects with coronary artery disease and apparently healthy individuals on biomaterials

On the basis of the clinical studies in patients with coronary artery disease (CAD) presenting an increased percentage of activated platelets, we hypothesized that hemocompatibility testing utilizing platelets from healthy individuals may result in an underestimation of the materials' thrombogenicity. Therefore, we investigated the interaction of polymer-based biomaterials with platelets from CAD patients in comparison to platelets from apparently healthy individuals. In vitro static thrombogenicity tests revealed that adherent platelet densities and total platelet covered areas were significantly increased for the low (polydimethylsiloxane, PDMS) and medium (Collagen) thrombogenic surfaces in the CAD group compared to the healthy subjects group. The area per single platelet-indicating the spreading and activation of the platelets-was markedly increased on PDMS treated with PRP from CAD subjects. This could not be observed for collagen or polytetrafluoroethylene (PTFE). For the latter material, platelet adhesion and surface coverage did not differ between the two groups. Irrespective of the substrate, the variability of these parameters was increased for CAD patients compared to healthy subjects. This indicates a higher reactivity of platelets from CAD patients compared to the healthy individuals. Our results revealed, for the first time, that utilizing platelets from apparently healthy donors bears the risk of underestimating the thrombogenicity of polymer-based biomaterials.

Polyglycerol coated polypropylene surfaces for protein and bacteria resistance

Polyglycerol coated polypropylene films were prepared in two steps by plasma bromination and grafting of polyglycerol. Films were characterized and their bioinertness against proteins and bacteria was investigated.

Surface-engineered nanogel assemblies with integrated blood compatibility, cell proliferation and antibacterial property: towards multifunctional biomedical membranes

This study presents the fabrication of multifunctional nanolayers on biomedical membrane surfaces by using LBL self-assembly of nanogels and heparin-like polymers.